Can you add a bit more details: Which Mac model and which specific Dell 30in are you using?
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patrix♦Feb 4 '13 at 5:02

You should be able to use the maximum resolution which I guess is either 2560*1440 or 2560 * 1600. But it is hard to tell if you don't give us the specific Mac model you use and what monitor model.
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AlexFeb 4 '13 at 7:55

1 Answer
1

OSX lion doesn't limit screen resolution, screen resolution is only limited by the graphics ability of the computer you're using and the screen res of the monitor you're using. If you're running lion then you have an intel chip mac. Older intel macs (pre 2009) with shared GMA video cards supported up to 1920×1200, however pretty much anything post 2009 will support up to 2560×1600 (which is what your Dell 30 inch monitor will be if its the current 3007WFP model)